SEGREGATION OF RECEPTIVE-FIELD PROPERTIES IN THE LATERAL GENICULATE-NUCLEUS OF A NEW-WORLD MONKEY, THE MARMOSET CALLITHRIX-JACCHUS

The lateral geniculate nucleus (LGN) in humans and Old-World monkeys i s dominated by the representation of the fovea in the parvocellular (P C) layers, and most PC cells in the foveal representation have red-gre en cone opponent receptive field properties. It is not known whether t hese features are both unique to trichromatic primates. Hen we measure d receptive field properties and the visuotopic organization of cells in the LGN of a New-World monkey, the marmoset Callithrix jacchus. The marmoset displays a polymorphism of cone opsins in the medium-long wa velength (ML) range, which allows the LGN of dichromatic (''red-green color blind'') and trichromatic individuals to be compared. Furthermor e, the koniocellular-interlaminar layers are segregated from the main PC layers in marmoset. allowing the functional role of this subdivisio n of the LGN to be assessed. We show that the representation of the vi sual field in the LGN is quantitatively similar in dichromatic and tri chromatic marmosets and is similar to that reported for macaque; the v ast majority of LGN volume is devoted to the central visual field. ON- and OFF-type responses are partially segregated in the PC layers so t hat ON responses are more commonly encountered near the external borde r of each layer. The red-green (ML) opponent cells in trichromatic ani mals were all located in the PC layers, and their receptive fields wer e within 16 degrees of the fovea. The koniocellular zone between the P C and magnocellular layers contained cells that receive excitatory inp ut from short wavelength sensitive cones (''blue-ON cells'') as well a s other nonopponent cells. These results suggest that the basic organi zation of the LGN is common to dichromatic and trichromatic primates a nd provide further evidence that ML and SWS opponent signals are carri ed in distinct subdivisions of the retinogeniculocortical pathway.